The present invention relates to liposomal bupivacaine compositions, useful for long term analgesia, which are prepared using an ammonium sulfate gradient loading procedure. The invention also provides methods of providing analgesia using such formulations. Also disclosed are liposome suspensions comprising xe2x80x98GMVxe2x80x99 (giant multivesicular) liposomes and methods for their preparation.
Boogaerts, J. G. etal.; Can J Anaesth 40(12):1201-1204 (1993).
Grant, G. J. et al., Reg Anesth 19(4):264-269 (1994).
Haran, G. et al., Biochem Biophys Acta 1151(2):201-215 (1993).
Kim, S. et al., U.S. Pat. No. 5,723,137 (1998).
Legros, F. et al., U.S. Pat. No. 5,244,678 (1993).
Mowat, J. J. et al., Anesthesiology 85(3):635-643 (1996).
Stewart, J.C.M., Anal. Biochem. 104, 10 (1959).
Liposomal encapsulation of local anesthetics has been shown to increase the duration of pain relief. Critical factors in the effectiveness of liposomal bupivacaine formulations include encapsulation of the maximal amount of drug, as well as a suitable release rate after injection. The primary disadvantages of previously described liposomal bupivacaine formulations (e.g. Boogaerts, Grant, Legros) are relatively inefficient drug entrapment and low drug/lipid ratios. This can lead to an undesirable deposit of fatty lipid material at the site of injection.
Recently, active loading of bupivacaine using a pH gradient was described (Mowat). A sodium citrate solution was used to establish a transmembrane gradient, and a drug/lipid ratio of 0.26 was achieved, higher than that obtained previously via standard xe2x80x9cpassivexe2x80x9d loading techniques (e.g. Legros). However, in Mowat, the extraliposomal pH was maintained at about 7.4, which potentially limits the amount of drug available for loading, as bupivacaine is poorly soluble at this pH.
The present invention includes, in one aspect, a method of producing prolonged analgesia by administering a liposomal bupivacaine composition. The composition is prepared by incubating a bupivacaine solution with a suspension of liposomes having a greater concentration of ammonium ions inside said liposomes than outside said liposomes. Such a suspension is prepared by (a) vortexing a lipid film with an aqueous solution of ammonium sulfate and (b) removing ammonium sulfate from the extraliposomal medium. Preferably, the aqueous solution has a concentration of about 250 mM ammonium sulfate, and, in the incubating step, the bupivacaine solution has a pH effective to prevent precipitation of the bupivacaine; typically about 6 or less.
The method of administering of the local anesthetic compositions may also comprise cooling of the administration site. Preferably, the skin of the subject is cooled to a temperature of about 22xc2x0 C. at the site of injection.
In a preferred embodiment, the liposomes are xe2x80x98GMVxe2x80x99 (giant multivesicular) liposomes. These are prepared by (a) vortexing a lipid film with an aqueous solution, such as a solution of ammonium sulfate, (b) homogenizing the resulting suspension to form a suspension of small unilamellar vesicles (SUV), and (c) repeatedly freeze-thawing said suspension of SUV in liquid nitrogen followed by water. Preferably, the freeze-thawing is repeated at least five times, and more preferably about ten times. The extraliposomal ammonium sulfate is then removed, e.g. by dialysis against normal saline. The GMV liposomes, and the disclosed method of preparation, provide additional aspects of the present invention. Preferably, the method of preparation includes encapsulating a therapeutic substance within the liposomes, e.g. by incubating a suspension of the liposomes with a solution of the therapeutic substance. Preferably, the substance is weakly basic, and the suspension of GMV liposomes has a greater concentration of ammonium ions inside the liposomes than outside the liposomes, as described above. Where the substance is bupivacaine, in the incubating step, the bupivacaine solution has a pH effective to prevent precipitation of the bupivacaine; typically about 6 or less.
The GMV-based liposomal bupivacaine compositions typically have a molar drug to lipid ratio, following removal of non-encapsulated bupivacaine, of at least 1.0. After further concentration of the resulting liposomal suspension by ultrafiltration or centrifugation, the a molar drug to lipid ratio is at least 0.5, preferably at least 1.0, and more preferably at least 1.5.